Temporary structures that need to be erected and folded for transportation and storage are often held together with hinges secured by integrated clamps. These clamps are more effective if they can be tightened and released quickly, easily and safely, and if the parts are not easily broken or lost.
This relates specifically to hinge clamps of folding bicycles and the like, but may also apply to hinge clamps that secure other structures. It may also apply to other clamps and other such mechanisms that have cranks or handles that can be spun by using a human finger or similar object inserted into a hole as an axis for spinning a crank.
Manufacturers offer bicycle frames that fold by way of hinges secured by clamps: two flanges or leaves are held together on one side by a pin running along the edges, and on the other side by a “V”-shaped clamp block or plate. The opening of the “V” is situated facing the pin and driven against surfaces along the open edges of the hinge. These surfaces are angled to mate with the angle of the “V”. The “V” plate is wedged onto the edges of the hinge by tightening a bolt running through it. This bolt is affixed to one of the two leaves perpendicular to the axis around which the leaves rotate by means of a threaded bore. The bolt head is equipped with a winged member that acts as a wrench to be turned by hand, driving the parts firmly together. Friction keeps the structure together despite the jarring conditions of a bicycle ride. The inner surfaces of the “V” are relieved along the center to concentrate clamping force on the outer edges of the hinge to resist bending loads and to clear brazed fillets connecting the hinges to the tubing of the bicycle frame.
Disadvantages of this design are that the operation of opening and closing the mechanism is unnecessarily frustrating, as are other aspects of its service as described below.
The clamp plate may rotate into the path of the closing hinge, preventing closure. The plate is not automatically retracted when released, and may prevent opening even when unbolted. The travel of the bolt away from the hinge is not limited, so time may be wasted turning the bolt unnecessarily. In transit, the bolt is unconstrained by friction, so vibration can work the bolt out of its bore, subjecting it to deformation or loss. The wing bolt requires dexterity to spin it efficiently with one finger, or it requires the user to realign her or his hands with the handles at every turn. In addition, a thief can assemble the bicycle using the built-in wrenches and flee upon it.
Aftermarket manufacturers have improved on the original design by elongating one arm of the “V” to engage the leaf holding the bolt, preventing its rotation. One manufacturer has instead engaged the steel leaf by inserting magnets into either end of one side of the clamp plate. This approach is unnecessarily costly to produce and has the disadvantage of collecting ferrous debris that might compromise the solidity of the joint.
Some upgrade kits provide springs positioned around the bolt between the clamp plate and the bolt-bearing leaf to urge them apart when the bolt is loosened. Springs may be long and narrow to fit in the groove around the bolt, or short and wide to fit outside of the hinges when closed. In either case, there is a tendency for the springs to be caught between the leaves when the bolt is tightened, causing damage to the spring and preventing a safe, secure joint.
Some kits provide longer bolts and nuts to prevent the bolts from turning more than necessary, allowing precise adjustment to limit turning time and loss or damage to bolts. Others provide screws than can be screwed into the end of the bolts. This is self-limiting, but may not provide the optimum range for every clamp.